Light beam measuring instruments are used in various situations. Among them are included, for instance, quality inspections of semiconductor products, in which semiconductor chips are inspected whether they are correctly formed or whether wiring is correctly made, and reverse engineering of a competitor's semiconductor circuit. An end-point detector (EPD) can be regarded as a type of light beam measuring instrument for determining whether etching reaches a desired depth or not by means of light reflected from the bottom surface of etching or of interference of the lights reflected from the top surface and reflected from the bottom surface of etching during etching by plasma or the like.